This document provides an overview of motherboard bus types with empahsis on compatibility and performance.
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Sunday, October 20, 2013 5:12 PM
Introduction And Overview Of Bus Types
The terms bus, bus type, and bus speed refer to the collection of wires and integrated circuits through which data is transmitted on a motherboard. Bus type defines the standard for how components transfer data, while bus speed defines how fast the components can transfer data.
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers. Early computer buses were parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same logical functionality as a parallel electrical bus. Modern computer buses can use both parallel and bit serial connections, and can be wired in either a multidrop (electrical parallel) or daisy chain topology, or connected by switched hubs, as in the case of USB.
A bus type's performance is measured in megahertz (Mhz) and gigahertz (Ghz). For example, the obsolete 80386 processor communicated with other peripheral devices using a 16-bit data bus. This meant that it could only transmit data using 16 bits at one time.
The Pentium processor transferred data through a 32 bit data bus which meant the system could transmit up to 32 bits of data at any given time. The Pentium processor could transmit data at a 100 Mhz bus speed. The early celeron processors could transfer at only 66 Mhz.
The Pentium IV processor also transfers data using a 32 bit data bus but at a much higher speed or frequency, often ranging in speeds from 400 Mhz to 800 Mhz depending on the processor and motherboard being used.
Expansion Card Bus Types
ISA (Industry Standard Architecture)
In the vintage days of computing, ISA (Industry Standard Architecture) was the standard bus type for all computer system in 80286, 80386, and 80486 based computers.
The design structure of ISA encompassed two types of connectors: AT and XT. AT slots were designed much longer than XT slots and supported a 16 bit data transfer pathway. This was the most common bus type in the PC arena during its evolution in 1984.
The ISA bus can be easily identified on a motherboard. It is the only brown colored connector and supports only 8-bit or 16-bit expansion cards. 16 bit ISA expansion boards have two connectors on on any particular card.
The most common ISA devices included these types of devices:
ISA transferred data using an 8 Mhz clock speed. Common ISA devices include sound and network cards and other peripherals that do not require a fast connection method for transferring data to the motherboard.
These machines employed the 80286 and 80386 processors and both used an eight bit data bus to transfer data. However, as the need for a faster method of transmission grew so did the need for a quicker bus type. As result of this demand, ISA was replaced by EISA (pronounced EEESA).
The evolution of a far more advanced processor design (most notably a 32-bit design structure was implemented in the 80386, 80486, and Pentium based processors), a faster bus type was a neccessity. Thus, EISA developed to accomodate these design changes.
Some 16 bit EISA boards were referred to as smart sensing boards in that they were backwards compatible with expansion boards that utilized an 8 bit EISA connector.
EISA was better suited for server based personal computers. EISA was capable of bus mastering the concept that allows components attached to the bus to talk to each other without bothering the CPU. Often compared to the SCSI interface, the computer could be sped up quite well through the use of the EISA bus. However, even though EISA was a better bus type it was still a very slow standard. As a result EISA became phased out by the PCI (Peripheral Component Interconnect) bus.
Like the ISA bus type the PCI bus connector is easily identifiable on a motherboard. It is the only white colored expansion slot. The Peripheral Component Interconnect (PCI) bus type which transfers data at a rate of thirty three megabytes per second (Mbps).
PCI quickly evolved to become a standard for peripheral devices. The PCI bus is more robust bus type and offers a higher performance rating for peripherals inside a personal computer. Although PCI is limited to five connectors, modern motherboards often support six through two seperate controller chips appropriately labelled the North and South bridge chipsets.
Modern motherboards supporting an ATX connection have between four and five PCI slots. Micro ATX mainbaords generally have a maximum of three PCI slots.
The most common PCI devices include the following:
With the introduction of the PCI bus type and Windows 95b, Microsoft introduced the concept known as Plug and Play technology. Plug and Play is a Windows 95 feature that permits for the automatic detection and installation of expansion cards and device drivers. As new cards are plugged into a computer system, Plug and Play allowed the automatic detection of certain configuration settings without the need for manual configurations to be implemented.
Originally planned, the first PCI standard ran at 33 Mhz but was later revised to run at a 66 Mhz speed to allow for increased peformance. The PCI bus running through a 32 bit data bus at 33 Mhz has a total throughput rate of 133 Mbps.
The Accelerated Graphics Port (often shortened to AGP) is a high-speed point-to-point channel for attaching a video card to a computer's motherboard, primarily to assist in the acceleration of 3D computer graphics. Since 2004 AGP has been progressively phased out in favor of PCI Express (PCIe). By mid-2009 PCIe cards dominated the market; AGP cards and motherboards were still produced, but OEM driver support was minimal.
PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe, is a computer expansion bus standard designed to replace the older PCI, PCI-X, and AGP bus standards. PCIe has numerous improvements over the aforementioned bus standards, including higher maximum system bus throughput, lower I/O pin count and smaller physical footprint, better performance-scaling for bus devices, a more detailed error detection and reporting mechanism (Advanced Error Reporting (AER), and native hot-plug functionality. More recent revisions of the PCIe standard support hardware I/O virtualization. The PCIe electrical interface is also used in a variety of other standards, most notably ExpressCard, a laptop expansion card interface. Format specifications are maintained and developed by the PCI-SIG (PCI Special Interest Group), a group of more than 900 companies that also maintain the Conventional PCI specifications. PCIe 3.0 is the latest standard for expansion cards that is available on mainstream personal computers.
Hard Drive And Optical Storage Bus Types
A hard disk drive transfers data through the internal IDE bus running at a 100 or 133 Mbps.
The fastest bus type on a motherboard is the connection between the processor and the Level 1 and Level 2 cache memory segments. The next fastest bus type links the processor with the secondary cache memory and main system memory also known as Random Access Memory (RAM).